For most durable goods, buying used can save a lot of money, especially in the case of autos, books, etc that rapidly depreciate in value after the initial sale, and can easily be found in good, often near new condition. The key is to buy from a trusted source with a good reputation (nudge nudge porcs...).
A good used-car buying tip I learned from a Porc in socal is to never pay license/tax/reg fees- bid down as far as you can, then refuse to close the sale without the dealer eating the costs. I saved about 1K this way buying a used Subaru from a dealer in Socal, one of the most expensive car markets. Note that you probably won't be lucky enough to do this with some dude selling it in a classified ad. Personally, I'd rather pay more to know what I'm getting, and have some kind of resource. That's just me. I paid 11K for a '01 Subaru Forrester, the asking price was originally 14,300 plus almost 1,000 in various state fees/taxes (15,300 total). I then spent about 450 in repairs (got a cracked windshield and a wheel bearing went out after about a month). The final cost after all that? about 11,450. So I saved myself 3,300 dollars, simply by being assertive.
Any other similar tips?